The present invention relates to a process for the complete replacement of particles in a vessel. The present invention relates in particular to a process for the complete replacement of catalyst particles used in the catalytic treatment of hydrocarbon oils such as demetallization, desulfurization and hydroconversion.
Since the feedstocks normally applied in oil processes, such as atmospheric and vacuum residues contain both metals such as nickel and vanadium as well as sulfur compounds, pre-treatments like (hydro)demetallization and (hydro)desulfurization are required to improve the quality of the feedstock prior to further conversion envisaged.
Catalysts play a very important role in hydroprocessing. Proper fluid flow is desired to make full use of a catalyst's particular properties. The life of a catalyst in residue hydroprocessing units is limited mainly due to the deposition of metals. Therefore, the catalyst has to be replaced at certain intervals by a fresh charge, the spent charge either being discarded or regenerated in a separate operation. It will be clear that the replacement of catalysts is carried out preferably as late as possible and, moreover, with a minimum down-time of the reactor concerned.
Quick catalyst replacement operations have been suggested in the art (e.g., British Patent Specification No. 1,500,213) and can be applied successfully, but they have to be carried out while the reactor concerned is taken off-stream and has to be depressurized and cooled down to ambient temperature.
On-stream replacement of catalyst charges can be carried out in moving-bed or bunker reactors (e.g., as described in British Patent Specification No. 1,331,935). The replacement occurs gradually and can be carried out continuously or periodically in bunker reactors equipped for mass-flow behavior of the catalyst charge. Mass-flow behavior of the solids present is normally obtained by resting the catalyst bed(s) on one or more conical supports having in the center a catalyst outlet duct leading to a sluicing system. Since catalyst replacement is carried out on-stream, the addition of fresh catalyst also requires the presence of a sluicing system.
It may be necessary for various reasons, however, to replace the complete charge of a catalyst or catalyst/carrier mixture present in a reactor while maintaining the reactor under operating conditions. This can be accomplished successfully when a number of reactors is operated in series, provided the reactor to be refilled with catalyst is temporarily disconnected from the supply and removal lines for the hydrocarbon feeds and effluents (e.g., British Patent Specification No. 2,014,058).
However, in carrying out this replacement operation, the main feed to the reactor has to be stopped and additional hot purge gas (e.g., hydrogen at a temperature of about 350.degree. C.) has to be purged through the reactor in order to prevent temperature excursions. The consequence of the use of a purge gas is, however, the presence of a complicated valve handling procedure around the reactor(s) concerned together with an additional hot purge gas flow circuit, in size and thoughput comparable with the recycle gas flow normally employed.
It has now been found that complete replacement under operating conditions of particles in vessels provided with inlet(s) and outlets can be accomplished without the presence of an additional purge gas circuit when the flow of feed through the vessel is controlled carefully during the catalyst withdrawal procedure.